[ticket/14717] Quote strings beginning with @ or % in yaml definitions

This type of use was deprecated in symfony 2.8 and will be dropped in 3.x.
Also see: https://github.com/symfony/symfony/pull/16285

PHPBB3-14717

tests/functional/fixtures/ext/foo/bar/config/services.yml

@@ -2,13 +2,13 @@ services:
     foo_bar.controller:
         class: foo\bar\controller\controller
         arguments:
-            - @controller.helper
-            - @path_helper
-            - @template
-            - @config
-            - @user
-            - %core.root_path%
-            - %core.php_ext%
+            - '@controller.helper'
+            - '@path_helper'
+            - '@template'
+            - '@config'
+            - '@user'
+            - '%core.root_path%'
+            - '%core.php_ext%'
 
     foo_bar.listener.permission:
         class: foo\bar\event\permission